It should be better than your 2600 purely because it's newer technology but I think you will struggle to get decent frames on the 320M. It's not a gaming video card. At best you could play it on low settings.
There is an article about the 13" Macbook Pro at http://www.anandtech.com/show/3759/mac-os-x-steam-performance-half-life-2-episode-2-still-slower-tha...
While it doesn't give a comparison between your 2600 it does show how that GPU performs with some Steam games. Keep in mind that the Macbook does have 2Gb of additional RAM though and this would probably give a slightly better performance.
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